linux-kernel - Re: [PATCH 01/10] mm/kasan: implement kasan_poison

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Message-ID: <CA+fCnZcvuXR3R-mG1EfztGx5Qvs1U92kuyYEypRJ4tnF=oG04A@mail.gmail.com>
Date: Thu, 4 Dec 2025 16:17:23 +0100
From: Andrey Konovalov <andreyknvl@...il.com>
To: Ethan Graham <ethan.w.s.graham@...il.com>
Cc: glider@...gle.com, andy@...nel.org, andy.shevchenko@...il.com, 
	brauner@...nel.org, brendan.higgins@...ux.dev, davem@...emloft.net, 
	davidgow@...gle.com, dhowells@...hat.com, dvyukov@...gle.com, 
	elver@...gle.com, herbert@...dor.apana.org.au, ignat@...udflare.com, 
	jack@...e.cz, jannh@...gle.com, johannes@...solutions.net, 
	kasan-dev@...glegroups.com, kees@...nel.org, kunit-dev@...glegroups.com, 
	linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-mm@...ck.org, lukas@...ner.de, rmoar@...gle.com, shuah@...nel.org, 
	sj@...nel.org, tarasmadan@...gle.com, Ethan Graham <ethangraham@...gle.com>
Subject: Re: [PATCH 01/10] mm/kasan: implement kasan_poison_range

On Thu, Dec 4, 2025 at 3:13 PM Ethan Graham <ethan.w.s.graham@...il.com> wrote:
>
> From: Ethan Graham <ethangraham@...gle.com>
>
> Introduce a new helper function, kasan_poison_range(), to encapsulate
> the logic for poisoning an arbitrary memory range of a given size, and
> expose it publically in <include/linux/kasan.h>.
>
> This is a preparatory change for the upcoming KFuzzTest patches, which
> requires the ability to poison the inter-region padding in its input
> buffers.
>
> No functional change to any other subsystem is intended by this commit.
>
> Signed-off-by: Ethan Graham <ethangraham@...gle.com>
> Signed-off-by: Ethan Graham <ethan.w.s.graham@...il.com>
> Reviewed-by: Alexander Potapenko <glider@...gle.com>
>
> ---
> PR v3:
> - Move kasan_poison_range into mm/kasan/common.c so that it is built
>   with HW_TAGS mode enabled.
> - Add a runtime check for kasan_enabled() in kasan_poison_range.
> - Add two WARN_ON()s in kasan_poison_range when the input is invalid.
> PR v1:
> - Enforce KASAN_GRANULE_SIZE alignment for the end of the range in
>   kasan_poison_range(), and return -EINVAL when this isn't respected.
> ---
> ---
>  include/linux/kasan.h | 11 +++++++++++
>  mm/kasan/common.c     | 37 +++++++++++++++++++++++++++++++++++++
>  2 files changed, 48 insertions(+)
>
> diff --git a/include/linux/kasan.h b/include/linux/kasan.h
> index 890011071f2b..cd6cdf732378 100644
> --- a/include/linux/kasan.h
> +++ b/include/linux/kasan.h
> @@ -102,6 +102,16 @@ static inline bool kasan_has_integrated_init(void)
>  }
>
>  #ifdef CONFIG_KASAN
> +
> +/**
> + * kasan_poison_range - poison the memory range [@addr, @addr + @size)
> + *
> + * The exact behavior is subject to alignment with KASAN_GRANULE_SIZE, defined
> + * in <mm/kasan/kasan.h>: if @start is unaligned, the initial partial granule
> + * at the beginning of the range is only poisoned if CONFIG_KASAN_GENERIC=y.

You can also mention that @addr + @size must be aligned.

> + */
> +int kasan_poison_range(const void *addr, size_t size);
> +
>  void __kasan_unpoison_range(const void *addr, size_t size);
>  static __always_inline void kasan_unpoison_range(const void *addr, size_t size)
>  {
> @@ -402,6 +412,7 @@ static __always_inline bool kasan_check_byte(const void *addr)
>
>  #else /* CONFIG_KASAN */
>
> +static inline int kasan_poison_range(const void *start, size_t size) { return 0; }
>  static inline void kasan_unpoison_range(const void *address, size_t size) {}
>  static inline void kasan_poison_pages(struct page *page, unsigned int order,
>                                       bool init) {}
> diff --git a/mm/kasan/common.c b/mm/kasan/common.c
> index 9142964ab9c9..c83579ef37c6 100644
> --- a/mm/kasan/common.c
> +++ b/mm/kasan/common.c
> @@ -570,3 +570,40 @@ bool __kasan_check_byte(const void *address, unsigned long ip)
>         }
>         return true;
>  }
> +
> +int kasan_poison_range(const void *addr, size_t size)
> +{
> +       uintptr_t start_addr = (uintptr_t)addr;
> +       uintptr_t head_granule_start;
> +       uintptr_t poison_body_start;
> +       uintptr_t poison_body_end;
> +       size_t head_prefix_size;
> +       uintptr_t end_addr;
> +
> +       if (!kasan_enabled())
> +               return 0;

Please move this check to include/linux/kasan.h; see how
kasan_unpoison_range() is implemented. Otherwise eventually these
checks start creeping into lower level functions and the logic of
checking when and whether KASAN is enabled becomes a mess.

> +
> +       end_addr = start_addr + size;
> +       if (WARN_ON(end_addr % KASAN_GRANULE_SIZE))
> +               return -EINVAL;
> +
> +       if (WARN_ON(start_addr >= end_addr))
> +               return -EINVAL;
> +
> +       head_granule_start = ALIGN_DOWN(start_addr, KASAN_GRANULE_SIZE);
> +       head_prefix_size = start_addr - head_granule_start;
> +
> +       if (IS_ENABLED(CONFIG_KASAN_GENERIC) && head_prefix_size > 0)
> +               kasan_poison_last_granule((void *)head_granule_start,
> +                                         head_prefix_size);

As I mentioned before, please rename kasan_poison_last_granule() to
kasan_poison_granule() (or maybe even kasan_poison_partial_granule?).
Here the granule being poisoned is not the last one.


> +
> +       poison_body_start = ALIGN(start_addr, KASAN_GRANULE_SIZE);
> +       poison_body_end = end_addr;
> +
> +       if (poison_body_start < poison_body_end)
> +               kasan_poison((void *)poison_body_start,
> +                            poison_body_end - poison_body_start,
> +                            KASAN_SLAB_REDZONE, false);
> +       return 0;
> +}
> +EXPORT_SYMBOL(kasan_poison_range);
> --
> 2.51.0
>